General and facile methods that permit precise control over the size and surface chemistry of micrometer-scale oil droplets broadly enable both fundamental studies of confined condensed phases (e.g., effects of confinement on order) as well as advance a range of promising technologies that revolve around control of dispersed phases (e.g., nano-materials, meso-materials, responsive materials, optical materials, filters, sensors, and opto-electronic technologies).
Previous studies have shown that emulsion droplets can be prepared by various techniques, such as photopolymerization, ultrasonication, shearing of droplets and subsequent crystallization fractionation, droplet break-off in a co-flowing stream (microfluidics), and dispersion polymerization. Although most of these approaches result in polydisperse emulsions, emulsion droplets with limited control over interfacial properties, or the formation of polymerized droplets, the microfluidic approach enables the preparation of monodisperse emulsion droplets with sizes larger than approximately 2 μm. Typical quantities of emulsion droplets prepared by microfluidics are on the order of 1-5 s−1 for a single junction. Recent, more elaborate multiple-device systems can produce 100-1000 particles s−1.
Liquid crystal (LC) materials are emerging as promising candidates for a range of sensing and interfacial applications. The ordering of LCs is highly sensitive to molecular-level events at the LC interface, enabling such interactions to be coupled to the orientational order of LCs, and thus leading to changes in the optical properties of the LC. For example, LCs respond to and amplify small changes in temperature, shear, electric or magnetic fields, or the structure of solid surfaces with which they are in contact. This qualifies LCs as “molecular magnifying glasses”, allowing events that occur at the nanoscale level to be observed at the spatial scale of the naked eye (and far-field optics) without the need for additional instrumentation. Recent reports have demonstrated that it is also possible to tailor the interfaces of LCs at aqueous interfaces in ways that provide control over the orientational order of the LC. For example, recent studies on thin films of supported LCs have demonstrated that orientational ordering transitions in LCs can be triggered by the presence of lipids, surfactants, proteins, and viruses. These changes in orientational order arise in part from coupling between the aliphatic tails of the adsorbed amphiphiles and the mesogens of the LC, and the nature and extent of these changes is influenced by the structure of the amphiphiles (e.g., tail length or head group structure) or by chemical or physical events in the aqueous phase that disrupt or perturb these assemblies (such as the binding or enzymatic action of a protein). Additionally, LC-based reporting offers potential advantages over conventional techniques because it does not require complex instrumentation or labels (enzymatic or fluorescent).
Despite these advances, there is a need for a general and scalable, highly parallel synthesis strategy that permits the formation of emulsions with fine control over their size (even below 1 μm) and surface chemistry. The manufacture of such emulsions would provide a route to new sensing and interfacial technologies, particularly biosensors based on monodisperse LC droplet emulsions.